Speaking at George Washington University on Tuesday, Kelly said the FBI currently has open terrorism investigations in all 50 states, and since 2013, there have been 37 ISIS-linked plots to attack the U.S.

Kelly called it an "unprecedented spike" in a type of violence that is "notoriously difficult to predict and control."

On "America's Newsroom" today, Rep. Peter King (R-NY) said it's becoming harder and harder to detect and stop homegrown terrorists.